1. Field of the Invention
This invention relates generally to drilling or mud motors used for drilling wellbores and more particularly to methods of making such motors.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes or wellbores are drilled by rotating a drill bit attached to a drill string end. A substantial proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes, to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth""s formations. Modern directional drilling systems generally employ a drill string having a drill bit at the bottom that is rotated by a motor (commonly referred to in the oilfield as the xe2x80x9cmud motorxe2x80x9d or the xe2x80x9cdrilling motorxe2x80x9d).
Positive displacement motors are commonly used as mud motors. U.S. Pat. No. 5,135,059, assigned to the assignee hereof, which is incorporated herein by reference, discloses one such mud motor. A typical mud motor includes a power section which contains a stator and a rotor disposed in the stator. The stator typically includes a metal housing which is lined inside with a helically contoured or lobed elastomeric material. The rotor is usually made from a suitable metal, such as steel, and has an outer lobed surface. Pressurized drilling fluid (commonly known as the xe2x80x9cmudxe2x80x9d or xe2x80x9cdrilling fluidxe2x80x9d) is pumped into a progressive cavity formed between the rotor and stator lobes. The force of the pressurized fluid pumped into the cavity causes the rotor to turn in a planetary-type motion. A suitable shaft connected to the rotor via a flexible coupling compensates for eccentric movement of the rotor. The shaft is coupled to a bearing assembly having a drive shaft (commonly referred to as the xe2x80x9cdrive subxe2x80x9d) which in turn rotates the drill bit attached thereto. Other examples of the drilling motors are disclosed in U.S. Pat. Nos. 4,729,675, 4,982,801 and 5,074,681.
As noted above, both the rotor and stator are lobed. The rotor and stator lobe profiles are similar, with the rotor having one less lobe than the stator. The difference between the number of lobes on the stator and rotor results in an eccentricity between the axis of rotation of the rotor and the axis of the stator. The lobes and helix angles are designed such that the rotor and stator lobe pair seal at discrete intervals. This results in the creation of axial fluid chambers or cavities which are filled by the pressurized circulating fluid. The action of the pressurized circulating fluid causes the rotor to rotate and precess within the stator.
The rotor typically is made of a material such as steel and has an outer contoured surface which is relatively easily to manufacture with precision. The stator, however, has an inner lobed surface and is made of an elastomeric material, typically by an injection molding process. The thickness of the elastomer varies with the contour of the lobes. Manufacturing of stators requires detailed attention to elastomer composition, consistency, bond integrity and lobe profile accuracy. The stators of relatively large mud motors can be several feet long. Because of the stator""s physical characteristics (length, lobe profile, etc.) and the precision required, stators are frequently made by joining smaller sections. Such manufacturing processes are time consuming, expensive and offer few flexibilities. Also, since the elastomeric layer is typically non-uniform, it exhibits uneven heat dissipation and wear characteristics.
Stators with relatively thin and uniform elastomeric layers tend to perform better and have longer operating lives than those of non-uniform elastomeric stators described above. In some applications, completely metallic stators or having a non-elastomeric layer, such as a ceramic layer, may be preferred.
The present invention addresses certain problems with the prior art methods of making mud motors and provides methods for manufacturing mud motors, wherein the stator is made as a continuous member with inner surface having a desired profile, which is then lined with a substantially uniform layer of a suitable material such as an elastomeric or ceramic material. The methods of the present invention are efficient and cost effective.
The present invention provides methods of manufacturing mud motors. The motor includes a stator and a rotor which is rotatably disposed in the stator. In one method, to form the stator, a mandrel whose outer surface substantially corresponds to the inverse of the desired inner profile of the stator is disposed inside a metal tubular member. The mandrel has a slightly tapered end for easy retrieval from the tubular member. The metal tubular member with the mandrel therein is placed between at least two rollers disposed on opposite sides of the tubular member. The rollers, while urging against the tubular member, rotate in opposite directions (one clockwise and the other counter-clockwise), thereby moving on the tubular member in the same direction. These rollers rotate back and forth thereby stroking over the tubular member. This stroking motion reduces the outer dimensions of the tubular member. The tubular member is rotated about its longitudinal axis while the rollers stroke. The process is continued until the inside of the tubular member attains the profile defined by the outer profile of the mandrel. After a section of the tubular member is formed, the tubular member is moved axially to form the next section The inside of the tubular member is then lined with a suitable material, such as an elastomer or a ceramic material. A suitable rotor having a desired outer lobed surface is then rotatably disposed in the stator to form the motor.
In an alternative method for manufacturing the mud motor, the stator is formed by compressing a tubular member by a plurality of continuously rolling rollers. A mandrel whose outer surface corresponds to the inverse of the desired inner profile of the stator is placed inside a metal tubular member. The mandrel has a slightly tapered surface for easy retrieval from the tubular member. A plurality of rollers are urged against the tubular member while rotating in a common direction, thereby rotating the tubular member in the direction opposite that of the rollers. This rolling action reduces the outer dimensions of the tubular member. The process is continued until the inside of the tubular member attains the desired profile.
In yet another method of forming a stator, a tubular member having therein a mandrel with an outer contoured surface is alternately pressed with a plurality of dies disposed around the tubular member""s outer surface, thereby reducing the outside dimensions of the tubular member. The process is continued until the inside surface of the tubular member attains the profile defined by the mandrel. The tubular member inside is lined with a suitable elastomer.
In still another method of making a mud motor, a mandrel is formed with a contoured outer surface that substantially corresponds to the inverse of the desired inner profile of the stator. The contoured outer surface of the mandrel is made of a frangible material, such as ceramic. The mandrel is designed to account for the load and shrinkage of the formed section of the stator. The mandrel is sprayed with a suitable metal to a desired thickness to form a tubular member. The mandrel is then removed from the tubular member. The resulting tubular member has the desired inside profile of the stator which is then lined with an elastomeric material.
In each of the methods described above, the elastomeric material is preferably injection molded over the inner surface of the tubular member. Alternatively, the rotor may have an outer elastomeric or ceramic layer or both the rotor and stator may have metal-to-metal contacting surfaces.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.